Metformin don't 'directly' affect your blood glucose - instead they are intended to make it easier for your own insulin to work properly, to deal with everything that insulin does for us. Odd things it needs to do like keeping your heat beating, your lungs. liver and lights etc, oooh and your brain working - as well as dealing with the food we eat, to provide the glucose to fuel every cell in the body - and STAY ALIVE ! LOL
If you take one tablet occasionally it will do absolutely nothing - it needs to build up gradually to an effective level in your body over time (usually over the first week to 3 weeks) and be kept 'topped up' by continuing to take it regularly thereafter to continue working for you.
Since the drug has gastric side effects - that's why it's advised to take it before eating. It was originally designed as a weight loss drug to (sort of, the non scientific explanation) make more carbohydrate pass through your guts quicker (and out the other end) before the body has time to lay it down as body fat. However - IT doesn't come out of the other end - and it's effects at making your body more sensitive to your insulin, continue on. They didn't know it would do this when it was first prescribed - and the blood glucose lowering effect was only discovered after it had been prescribed to a good sample of overweight people, a number of whom were (unsurprisingly) also diabetic ! It has the BG lowering effect on people who are thin, too - so what an unexpected bonus. More recently - they are also finding as they do more and more research, that it helps other bodily processes too for people, even if you aren't diabetic, which is great.
It's an all round good guy - but amongst those who know it well - there's a good reason why it's often dubbed 'Met fartin' LOL
Hope that explains it a bit.
